This invention relates to the manufacture of color cathode ray tubes and more particularly to exposure means for aiding the fabrication of the basic structure of a patterned color screen on the viewing panel of a color cathode ray tube.
Cathode ray tubes capable of presenting multi-colored display imagery, such as those employed in color television applications, conventionally utilize face panels having viewing areas whereupon patterned screens are interiorly disposed; such screens being comprised of repetitive groupings of related cathodoluminescent phosphor materials. For example, in certain types of the well-known shadow mask tube construction, the screen pattern is conventionally composed of a vast multitude of similarly-shaped phosphor elements definitively separated by relatively small interstitial spacings purposely provided to enhance the color purity of the imagery by reducing the possibility of happenstantial electron excitation of adjacent elements.
To enhance brightness and contrast of the color screen image, an opaque light-absorbing material is often disposed in the interstitial spacing to effect a basic multi-windowed construction or webbing for defining the respective phosphor elements of the screen pattern. It is important that this graphite coating, applied to the interior of the panel during formation of the webbing, be subsequently removed from the sealing edge of the panel and for a fixed distance therefrom up the sidewall of the panel to provide a peripherally defined outline of the basic windowed structure. Any residual graphite adhering to the sealing edge of the panel interferes with the subsequent formation of the hermetic jointure between the panel and the funnel. In addition, since graphite is electrically conductive, any particles thereof remaining on or around the mask supporting studs embedded in the panel sidewall may be dislodged by movement of the mask locating members and thereby constitute a potential source of deleterious electrical shorts and arcing in the finished tube.
It is a desideratum from both aesthetic and quality considerations, that the basic windowed webbing of the screen structure be framed by a circumscribing opaque peripheral area having a smoothly defined trim line along the upper portion of the panel sidewall in the region proximal to the viewing area. Such definition is desired as subsequent aluminizing of the completed screen should overlay all of the screen area including the opaque periphery therearound. In the completed panel, the remainder of the sidewall below the trim line, and the sealing edge therearound should be free of both graphite and aluminum.
In one procedure for fabricating the basic windowed webbing portion of the screen structure, the interior of the panel is coated with a negative photosensitized material, such as a dichromated aqueous-alcohol solution of polyvinyl alcohol whereupon the viewing area of the panel is then plurally exposed by actinic radiation emanating from discretely positioned sources and directed through the openings of the shadow mask to polymerize a similarly-shaped pattern in the sensitized coating therebeneath. The exposed coating is then developed to remove the unpolymerized areas, thereby providing a basic polymerized screen format pattern surrounded by a web pattern of substantially bare glass. The whole of the interior of the panel is then overcoated with a graphite material and subsequently treated with an appropriate degrading agent, to effect an effervescent degradation of the polymerized portions of the patterned screen format, such degradation also loosening the graphite overcoating the polymerized areas. The loosened materials are then removed by development to produce an opaque interstitial web defining multitudinous bare glass windows wherein the respective phosphor elements of the screen structure are subsequently disposed. Since the aforedescribed degradation affects polymerized areas which are solely related to the viewing area of the panel, the balance of the panel including the sidewall, the mask positioning studs therein, and the sealing edge therearound, remain coated with graphite.
By one conventional procedure, the extraneous graphite coating is removed from the panel edge and a portion of the adjacent sidewall by immersing the sealing edge of the panel into a suitable acid solution to chemically etch away the undesired graphite coating, trimming it to the level determined by the depth of immersion. While this method efficiently removes the graphite from the sealing edge, it manifests two deleterious results. First, by immersing the panel into the acid solution far enough to remove graphite from the mask positioning studs, there is the inherent danger of splashing acid upon the alreadyformed windowed webbing of the basic screen structure, thereby destroying the constancy of the pattern, and rendering the panel unusable for tube fabrication. If only shallow immersion of the panel sidewall is employed, it is necessary to clean the mask positioning studs by manual techniques, the results of which, are not uniformly consistent. Secondly, the acid immersion process of removing the graphite coating from the panel sidewall often results in the formation of a trim line having a jagged edge. Aside from the undesirable aesthetic appearance of panels trimmed by this procedure, there is a possibility that upon aluminizing, portions of the jagged graphite trim line may exhibit poor adherence and eventually produce loose particles within the tube.
An improvement has been developed in the procedure for fabricating the basic multi-windowed patterned portion of a color cathode ray tube screen structure formed on the interior of the face panel. This improvement involves additional photoexposure of the panel after the latent polymerized image of the window pattern has been formed on the viewing area and before development of that window pattern has been effected. This additional exposure is directed to that region of the sensitized coating adhered to the sidewall portion of the panel. To accomplish the sidewall exposure, the panel is positioned to facilitate orientation of an areal internal shielding member within the coated panel at a location substantially intermediate the viewing area and the mask positioning means embedded in the panel sidewall. This shielding member provides protection for the priorly-formed latent imagery of the window pattern during subsequent exposure of the pattern. At least one emitter of radiant energy is oriented relative to the shielding member and the sidewall of the panel, whereupon exposure of only that portion of the sensitized coating disposed on the sidewall is effected by a directed flood of exposure energy to provide a defined peripheral band of polymerized coating on the wall. The shielding member and the secondary emission means are thence removed from the vicinity of the panel, whereupon the diversely exposed areas of the coating on the interior of the panel are developed to remove the unexposed portions thereby providing a pattern of polymerized window areas on the viewing portion and a polymerized band on the panel sidewall. The interior of the panel is then conventionally overcoated with an opaque graphite material, and treated with a degrading agent to loosen and remove therefrom the polymerized material, along with the graphite deposited thereon. Removal of these degradation materials provides the interstitially defined window webbing of the basic screen structure on the viewing area which is circumscribed by a substantially bare glass sidewall wherein the mask positioning means and the terminal sealing edge therearound are free of coatings and particulate materials.